'\" te
.\"  Copyright 1989 AT&T
.\" Copyright (c) 2002 Sun Microsystems, Inc. All Rights Reserved
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.TH SRV 9E "Mar 15, 2019"
.SH NAME
srv \- service queued messages
.SH SYNOPSIS
.LP
.nf
#include <sys/types.h>
#include <sys/stream.h>
#include <sys/stropts.h>
#include <sys/ddi.h>
#include <sys/sunddi.h>



\fBint prefix\fR\fBrsrv\fR(\fBqueue_t *\fR\fIq\fR);	/* read side */
.fi

.LP
.nf
\fBint prefix\fR\fBwsrv\fR(\fBqueue_t *\fR\fIq\fR);	/* write side */
.fi

.SH INTERFACE LEVEL
.LP
Architecture independent level 1 (DDI/DKI). This entry point is required for
\fBSTREAMS\fR.
.SH ARGUMENTS
.ne 2
.na
\fB\fIq\fR\fR
.ad
.RS 5n
Pointer to the \fBqueue\fR(9S) structure.
.RE

.SH DESCRIPTION
.LP
The optional service \fBsrv()\fR routine may be included in a \fBSTREAMS
\fRmodule or driver for many possible reasons, including:
.RS +4
.TP
.ie t \(bu
.el o
to provide greater control over the flow of messages in a stream;
.RE
.RS +4
.TP
.ie t \(bu
.el o
to make it possible to defer the processing of some messages to avoid depleting
system resources;
.RE
.RS +4
.TP
.ie t \(bu
.el o
to combine small messages into larger ones, or break large messages into
smaller ones;
.RE
.RS +4
.TP
.ie t \(bu
.el o
to recover from resource allocation failure. A module's or driver's
\fBput\fR(9E) routine can test for the availability of a resource, and if it is
not available, enqueue the message for later processing by the \fBsrv()\fR
routine.
.RE
.sp
.LP
A message is first passed to a module's or driver's \fBput\fR(9E) routine,
which may or may not do some processing. It must then either:
.RS +4
.TP
.ie t \(bu
.el o
Pass the message to the next stream component with \fBputnext\fR(9F).
.RE
.RS +4
.TP
.ie t \(bu
.el o
If a \fBsrv()\fR routine has been included, it may call \fBputq\fR(9F) to place
the message on the queue.
.RE
.sp
.LP
Once a message has been enqueued, the \fBSTREAMS \fRscheduler controls the
service routine's invocation. The scheduler calls the service routines in
\fBFIFO \fRorder. The scheduler cannot guarantee a maximum delay \fBsrv()\fR
routine to be called except that it will happen before any user level process
are run.
.sp
.LP
Every stream component (stream head, module or driver) has limit values it uses
to implement flow control. Each component should check the tunable high and low
water marks to stop and restart the flow of message processing. Flow control
limits apply only between two adjacent components with \fBsrv()\fR routines.
.sp
.LP
\fBSTREAMS \fR messages can be defined to have up to 256 different priorities
to support requirements for multiple bands of data flow. At a minimum, a stream
must distinguish between normal (priority zero) messages and high priority
messages (such as \fBM_IOCACK\fR). High priority messages are always placed at
the head of the \fBsrv()\fR routine's queue, after any other enqueued high
priority messages. Next are messages from all included priority bands, which
are enqueued in decreasing order of priority. Each priority band has its own
flow control limits. If a flow controlled band is stopped, all lower priority
bands are also stopped.
.sp
.LP
Once the \fBSTREAMS \fRscheduler calls a \fBsrv()\fR routine, it must process
all messages on its queue. The following steps are general guidelines for
processing messages. Keep in mind that many of the details of how a \fBsrv()\fR
routine should be written depend of the implementation, the direction of flow
(upstream or downstream), and whether it is for a module or a driver.
.RS +4
.TP
1.
Use \fBgetq\fR(9F) to get the next enqueued message.
.RE
.RS +4
.TP
2.
If the message is high priority, process (if appropriate) and pass to the
next stream component with \fBputnext\fR(9F).
.RE
.RS +4
.TP
3.
If it is not a high priority message (and therefore subject to flow
control), attempt to send it to the next stream component with a \fBsrv()\fR
routine. Use \fBbcanputnext\fR(9F) to determine if this can be done.
.RE
.RS +4
.TP
4.
If the message cannot be passed, put it back on the queue with
\fBputbq\fR(9F). If it can be passed, process (if appropriate) and pass with
\fBputnext()\fR.
.RE
.SH RETURN VALUES
.LP
Ignored.
.SH SEE ALSO
.LP
.BR put (9E),
.BR bcanput (9F),
.BR bcanputnext (9F),
.BR canput (9F),
.BR canputnext (9F),
.BR getq (9F),
.BR nulldev (9F),
.BR putbq (9F),
.BR putnext (9F),
.BR putq (9F),
.BR qinit (9S),
.BR queue (9S)
.sp
.LP
\fIWriting Device Drivers\fR
.sp
.LP
\fISTREAMS Programming Guide\fR
.SH WARNINGS
.LP
Each stream module must specify a read and a write service \fBsrv()\fR routine.
If a service routine is not needed (because the \fBput()\fR routine processes
all messages), a \fBNULL\fR pointer should be placed in module's
\fBqinit\fR(9S) structure. Do not use \fBnulldev\fR(9F) instead of the
\fBNULL\fR pointer. Use of \fBnulldev\fR(9F) for a \fBsrv()\fR routine can
result in flow control errors.
